Acoustic energy, such as ultrasound, penetrates well through soft tissues and, due to its short wavelengths, can be focused to spots with dimensions of a few millimeters. As a consequence of these properties, acoustic energy can and has been used for a variety of diagnostic and therapeutic medical purposes, including ultrasound imaging and non-invasive surgery of many parts of the body. For example, by heating diseased (e.g., cancerous) tissue using ultrasound, it is often possible to ablate the diseased portions without causing significant damage to surrounding healthy tissue.
The noninvasive nature of ultrasound surgery is particularly appealing for the treatment of brain tissue. Moreover, coherent, non-invasive focusing of ultrasound through the human skull has been considered as a tool for targeted drug delivery to the brain, improved thrombolytic stroke treatment, blood flow imaging, the detection of internal bleeding, and tomographic brain imaging. However, the human skull has been a barrier to the clinical realization of many of these applications. Impediments to transcranial procedures include strong attenuation and the distortions caused by irregularities in the skull's shape, density, and thickness, which contribute towards destroying the ultrasound focus and/or decreasing the ability to spatially register received diagnostic information.
Conventional approaches for overcoming the transskull focusing difficulties described above involve the use of receiving probes designed for catheter insertion into the brain to measure the amplitude and phase distortion caused by the skull; corrections to the focus are then made by adjusting the ultrasound beam emitted from a transducer array. Alternatively, a completely noninvasive approach uses imaging information (e.g., X-ray computed tomography (CT) or MRI volumetric images), rather than receiving probes, to estimate the thickness, density and geometry of the skull surfaces and to predict the wave distortion caused thereby.
While the conventional approaches may partially improve the ultrasound focus, predicting the effects on the ultrasound beam traversing the skull remains challenging because of the nature of the skull and its multiple-layered internal structure, which varies from patient to patient; this limits the effectiveness and efficiency of transskull ultrasound treatment using conventional approaches. As a result, patients having skull structures that result in poor ultrasound focusing properties may get limited benefit from the ultrasound therapy in compliance with safety standards that limit deliverable energy levels. Accordingly, there is a need for an approach that predicts the effectiveness of ultrasound treatment for individual patients, improves ultrasound focusing properties, and effectively delivers ultrasound energy to a target through individual patients' skulls.